Musannaf Ibn Abi Shayba 22:160

Shababa ibn Sawwar narrated to us, he said: Shu'bah narrated to us, from Alqama ibn Marthad, he said: From Sa'id ibn Ubayda, from Abu Abd al-Rahman, from Uthman, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The best of you is he who learns the Quran and teaches it."